Joist Hangars

Discussion in 'Builders' Talk' started by Duncan Kershaw, Oct 5, 2018.

  1. Duncan Kershaw

    Duncan Kershaw New Member

    Hello All

    My first post so please be gentle :)

    I'm building a store room extension to a detached double garage. Its dense concrete block construction, 5m x 4m and will have a warm flat roof. I'm planning to run my 75x200 joists from the existing single skin brick wall of the attached garage across to my "new" extension wall.

    I had planned to install a wall plate (47x100mm) on the concrete blocks, then install the joists on top. However, this would have pushed the roof height about 60cm over the 2.5M planning restrictions so I'm having to save roof height where I can. My thinking is to use Masonary Rapid Build hangars like these; (https://www.secfixings.co.uk/product/rbjhi-rapid-build-joist-hangers/) and hang them down from the top course of blocks, reducing the roof height by 200mm (putting me well within planning restrictions)

    My question - Can I install these rapid build hangars on top of a wooden wall plate secured to the top course of blocks? I want to do this to add a little bit of lost ceiling height due to not installing the hangars on the top course of blocks.

    Apologies for the convoluted question...

    Any insight much appreciated.
     
  2. Jord86

    Jord86 Well-Known Member

    Just omit the 4x2 wallplate and sit the hangers on a course of brick commons instead.
     
  3. Duncan Kershaw

    Duncan Kershaw New Member

    OK thanks
     
  4. stevie22

    stevie22 Active Member

    Those are big timbers for a 4m span, especially if you're tight for height.
     
  5. Richard_

    Richard_ Active Member

    The building regs tables require 200 depth for a 4m span, it's because snow loading is high for a flat roof. A copy of the flat roof table is at the bottom of this page http://www.home-extension.co.uk/tech2.html
     
  6. stevie22

    stevie22 Active Member

    A lot of the stuff Richard refers to above is old.

    We use TRADA tables now which can be found on the web easily enough.

    You say warm deck is what you're going with but you could consider cold if height is a problem as it is generally thinner overall.

    Alternatively look at using shallower timbers at closer centres to reduce the depth.

    Either way 8x3 way over what you need.
     
  7. Richard_

    Richard_ Active Member

    Gravity hasn't changed recently. ;)

    To be fair, those tables are based on C16 and nowadays timber is often C24. Other changes with EC5 make a marginal difference.

    So, using C24 could get you down to 170x47 spanning 4.2m at 400 c/c
     
  8. Duncan Kershaw

    Duncan Kershaw New Member

    Thanks for all the messages. Some replies below;

    The 75x200 joists will be spanning around 5M. I thought the span tables indicated I'd need these joists. I was planning on a 450mm gap. I've already purchased these so I'm stuck with them now. Could I increase the gap maybe?

    I decided on a cold roof, so it now fits with 2.5M.

    Question on ventilation. To join my work room to the garage, I've removed the soffits on one wall from the garage and will be hanging the joists of this wall and tieing in the pitch to flat roof (flat rubber roof). For ventilation, I was wondering if I can vent from the extension in the garage, as this still has three other walls with vents. I suppose the alternative is added vents where the pitch roof meets the flat roof, something I'd rather not do if possible for risk of leaks. See my childlike sketch for reference :)
     

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